1. Field of Invention
The present invention relates to a liquid crystal display device and electronic equipment, and specifically, it relates to a technique to produce a display with a high contrast and a wide viewing angle in a transreflective liquid crystal display device that produces displays both in reflective mode and transmissive mode.
2. Description of Related Art
Certain transreflective liquid crystal display devices working both in reflective mode and transmissive mode are known as liquid crystal display devices. Among such related art transreflective liquid crystal display devices, one including an upper substrate, a lower substrate, and a liquid crystal layer held between the upper and lower substrates and having a reflective film on an inner surface of the lower substrate has been proposed. In this device, the reflective film includes a film of metal, such as aluminum, has a window for optical transmission and functions as a transreflective plate. In reflective mode in this device, extraneous light incident from the upper substrate passes through the liquid crystal layer, is reflected by the reflective film on the inner surface of the lower substrate, passes through the liquid crystal layer again, exits from the upper substrate and thereby contributes to display. In contrast, in transmissive mode, light emitted from a backlight and incident from the lower substrate enters from the window of the reflective film, passes through the liquid crystal layer, exits from the upper substrate to the outside and thereby contributes to display. Of regions where the reflective film is arranged, a region of the window constitutes a transmissive display region, and the other region constitutes a reflective display region.
However, related art transreflective liquid crystal display devices have a narrow viewing angle in transmissive display. This is because these devices have the transreflective plate on the inner surface of the liquid crystal cell for avoiding parallax and must thereby produce reflective display by using only one deflection plate arranged on a side facing an observer. Owing to this constraint, they have a small degree of freedom in optical design. To solve this problem, Jisaki et al. propose a liquid crystal display device using a vertically aligned liquid crystal in Development of transreflective LCD for high contrast and wide viewing angle by using homeotropic alignment”, M. Jisaki et al., Asia Display/IDW'01, p. 133–136(2001). The device has the following three features.
(1) The device employs “vertical alignment (VA) mode” in which a liquid crystal having negative dielectric anisotropy is aligned vertically to substrates and tilts upon application of a voltage.
(2) It employs a “multi-gap structure” in which a transmissive display region and a reflective display region have different thicknesses in the liquid crystal layer (cell gaps) (see, for example, Japanese Unexamined Patent Application Publication No. H 11-242226).
(3) It employs a “domain division structure” in which the transmissive display region is an equilateral octagonal, and a protrusion is arranged in the center of a transmissive display region on a counter substrate so as to allow the liquid crystal to tilt in eight directions in this region.
However, Jisaki et al. never discloses any configuration to control the tilt direction of the liquid crystal molecules in the reflective display regions, although the tilt direction of the liquid crystal molecules in the transmissive display regions is controlled by the use of protrusions. In the reflective display regions, therefore, the liquid crystal molecules tilt in random directions, which induces discontinuous lines called “disclination” at the interfaces between different regions where liquid crystal molecules have different alignments, thus causing, for example, afterimages. In addition, regions where the liquid crystal molecules have different alignments show different viewing angle characteristics. Thus, rough, spotting uneven display is seen when the liquid crystal display device is viewed from an oblique direction.
Providing such a multi-gap structure in transreflective liquid crystal display devices is very effective to match the electro-optical characteristics (transmittance-voltage characteristics, reflectance-voltage characteristics) between the transmissive display regions and the reflective display regions. This is because the light passes through the liquid crystal layer twice in the reflective display regions, whereas it passes through the liquid crystal layer only once in the transmissive display regions.
FIGS. 12(A) and 12(B) illustrate effects of the multi-gap structure on the electro-optical characteristics (transmittance-voltage characteristic and reflectance-voltage characteristic). FIG. 12(A) shows the electro-optical characteristics when the transmissive display regions and the reflective display region have the same cell gap. Such a liquid crystal display device having no multi-gap structure has such a reflectance-voltage characteristic that a reflectance excessively sharply drops with an increasing voltage in reflective display and may induce display problems, such as decreased transmittance or the inversion of halftones in reflective display, unless the transmissive display regions and the reflective display regions are driven at different voltages. In contrast, FIG. 12(B) shows the electro-optical characteristics when the transmissive display regions have a cell gap about two times as large as the cell gap of the reflective display regions. By employing such a multi-gap structure the reflectance-voltage characteristics in reflective display substantially match the transmittance-voltage characteristics in transmissive display, and the reflective display regions and the transmissive display regions can be driven at the same voltage.
However, providing the multi-gap structure may induce a decreased substantial aperture, since liquid crystal molecules in step regions are resistant to move. The step regions allow the cell gaps different. In addition, its manufacturing process requires extra photoprocesses for the formation of the multi-gap structure, inviting increased cost.